1. Field of the Invention
This invention relates to a method for describing an electromagnetic signal generated by eddy currents in a conductive material.
Such a method can be used inter alia in determining properties of an electrically conductive measuring object composed of that material. This invention relates in particular to a method for determining properties of an electrically conductive measuring object, wherein:
a. utilizing at least one transmitting antenna, an electromagnetic field changing over time is emitted to the measuring object for generating eddy currents in the object; PA1 b. utilizing at least one receiving antenna, the electromagnetic signal generated by the eddy currents is detected; and PA1 c. on the basis of the detected electromagnetic signal, the properties of the measuring object are determined.
This invention further relates to an apparatus for practising such a method.
2. Description of Related Art
Such a method and apparatus are known from, for instance, U.S. Pat. No. 4,843,319. In this known method and apparatus, by means of a transmitting coil, a pulsated electromagnetic field is generated in the material of the measuring object. This gives rise to time-dependent eddy currents in the material. These eddy currents are detected by means of a receiving coil. The eddy currents, which change with time, cause a changing magnetic flux through the receiving coil, so that an induction voltage prevails across the receiving coil. Utilizing an amplifier, this changing induction voltage can be registered as a function of time. Thus, the electromagnetic signal generated by the eddy currents is detected as a function of the time t.
With the known apparatus, it is stated that the time-dependent behavior of the signal for small times t is determined by a constant logarithmic rate of decay of about 1.5. In other words, the received signal can be described with a signal V(t), to which the following relation applies: d(1n V)/d(1n t)=-1.5.
Beyond a certain critical time, designated .tau. and which is directly proportional to the square of the thickness of the surface of the material of the object under examination, so that .tau.=cd.sup.2, the logarithmic rate of decay falls to a lower value which equals A-2.171n(t). In other words, the following applies where t is greater than .tau.: d(1n V)/d(1n t)=A-2.171n(t). Here A is determined by the material properties and the geometry of the measuring object. Accordingly, before the thickness of the measuring object can be determined, first the constants c and A are to be determined. Determining the constants c and A is carried out through two measurements on two different test objects of the same material but having different, homogeneous wall thicknesses. This means that for carrying out the method, at all times two mutually different test specimens are to be at hand. Further, in this known method and apparatus, from a single measurement, only the homogeneous wall thickness of the material can be computed. Furthermore, the apparatus is limited to the use of a receiving coil for measuring the signal.